Slab Diffracted S Waves

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Inversion of P wave arrival times has found that lithospheric slabs frequently penetrate beyond the cutoff in the seismicity evident in Wadati-Benioff zones. Synthetic seismograms are calculated for teleseismic S waves radiated by a deep focus earthquake occurring within a slab that penetrates to 1200 km depth. The synthetics demonstrate a phenomenon of slab diffraction that acts to lengthen the apparent pulse width of the S wave. The slab diffraction is observed in all azimuths on the dipping side of the slab evolving into a prominent secondary phase along strike, and rapidly decaying on the side opposite to the dip. Broadband S waves are shown, which suggest the existence of slab diffraction in data. Amplitude ratios used in studies of mantle attenuation, D" structure, and core structure may also be affected by the focusing and defocusing effects of the high velocity perturbation of the slab. It is proposed to use a WKBJ/Gaussian beam method to synthesize the effects of slab diffraction in three-dimensional models in an effort to further constrain and invert for slab structure. An investigation will also be made of the contaminating effects of slab structure in other studies of mantle and core elastic and anelastic structure that use waveforms and amplitudes of body waves from deep focus earthquakes.